In the past, a communication system in which a transmission unit (master) and communication terminals (slaves) are connected to a transmission path and the respective communication terminals communicate with the transmission unit has become popular. As an instance of such a communication system, there has been proposed a system in which the transmission unit periodically monitors states of the respective communication terminals. In this system, upon acknowledging a change in a state of one communication terminal, the transmission unit sends a signal to another communication terminal to perform a processing corresponding to the change in the state (see patent documents 1 to 3).
However, in the communication with the above configuration, the communication terminals consistently communicate with each other through the transmission unit, and the transmission unit polls the communication terminals. Thus, a communication speed is relatively low. For example, this communication system is unsuitable for transmitting information with a relatively large data amount such as an analog quantity. Moreover, as for the aforementioned communication system, when the transmission unit malfunctions, the whole system halts. Hence, the communication system has low system reliability.
In view of this, there has been proposed a communication system designed as a hybrid of an existing communication system designed to allow communication terminals to communicate with each other through a transmission unit and a communication system designed to allow communication terminals to directly communicate with each other via peer-to-peer (P2P). In this hybrid communication system, a transmission path is shared by a first communication terminal communicating with the transmission unit and second communication terminals directly communicating with each other. Thus, the second communication terminal can be easily added on the existing communication system. The first communication terminal performs communication using a first communication signal (first protocol signal) which is repeatedly sent from the transmission unit, and the second communication terminal performs communication using a second communication signal (second protocol signal) which is superimposed on the first communication signal.
As for a general communication system, to extend the transmission path or to increase the number of terminals connected to the transmission path, relays are interposed between plural transmission paths. For example, a first transmission path and a second transmission path are connected to each other via a relay, and the relay relays a signal inputted from one transmission path of the first transmission path and the second transmission path to the other transmission path.
As for the communication system using the first communication signal and the second communication signal, with connecting a relay device for relaying the second communication signal in parallel with the relay in addition to provision of the relay for relaying the first communication signal, the first communication signal and the second communication signal can be relayed (see patent document 4). This relay device includes an extraction unit (extracting means) for extracting the second communication signal from a signal transmitted through one transmission path, a shaping unit (shaping means) for performing waveform shaping on this second communication signal, and a superimposing unit (superimposing means) for superimposing the second communication signal on the first communication signal on the other transmission path to send the second communication signal to the other transmission path.
Further, patent document 4 discloses that, to enable relaying the second communication signal from the first transmission path to the second transmission path and relaying the second communication signal from the second transmission path to the first transmission path, each of pairs of the extraction units, the shaping units, and the superimposing units is provided.